Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. One or more streamer cables containing acoustic seismic receivers are deployed into the water behind a vessel, and one or more sources may be towed by the same or different vessel. Less than perfect knowledge of the actual positions of the source at the time of firing and receivers at the time of arrival of reflected seismic waves may result in less than acceptable seismic data.
Most marine seismic surveys are acquired in straight lines and with parallel streamers that have constant separation. Thus most acoustic distance measuring systems have a fixed acoustic range length expectation. This length is used to set up the transmitter/receiver pairs in a configuration file, sometimes referred to as a “set-up file” of nominal separations, which is used to control the acoustic devices.
Of all previously known acoustic ranging systems employed in seismic data acquisition, only systems employing intrinsic ranging by modulated acoustics (IRMA) are integrated inline to the streamer. Historically all other acoustic ranging systems are provided by a third party in the sense that they are adapted to various streamers by attaching externally to the coil lines and are not tightly integrated with the software that transforms the acoustic and other positioning measures to coordinates for source and receivers in the seismic acquisition spread. These third party acoustic ranging systems can have difficulty if the relation between acoustic transmitting and receiving units change significantly in relation to the set-up file of nominal separations.
When any particular transmitter/receiver pair changes their relative position beyond the distance constraints that apply for the acoustic system, the relative position measurement may be lost. This may occur for example during network changes from straight line to curved line, in areas where the current changes feather or when coil shooting with varying radii of curvature. If this continues for a large number of measurements, the network quality decreases, and may lead to operational down time (also called non-productive time) if the relative positioning specifications are not met.
It is known in some instances to have the user manually update the approximate separations between transmitter and receiver in order to keep the acoustic ranging system functioning (this method may be referred to as manual range tracking). This is critical since many of these systems must separate the measurement times by time sharing transmissions in order for the transmissions not to interfere with each other. This constraint limits the length of the acoustic recording time and thus the range length and optimum transmitter/receiver pairs.
In periods when the transmitter/receiver pairs become poorly matched due to relative change in separation, acoustic range information may be missing from the network solution. In this case, there is a dependency on compass measures to control the crossline position estimate, while the inline acoustics are not so sensitive to streamer dynamics and will continue to track.
Another known method for range tracking is use of signal-to-noise ratio (“s/n”) as a guide for how the range is changing. This method depends on both a good s/n and no competing signal from reflections. Reflecting surfaces can be the sea bottom, sea surface, or other density interface. A diabolical situation is when there is refraction of most of the direct acoustic signal making it weak compared to a signal reflection that is not different from the direct by more than the record length. In this case the reflection gives the best s/n and can cause a tracking method to lock onto the reflection signal rather than the direct signal.
Range tracking has been used for acoustic systems such as IRMA to reduce the cpu needed to correlate ranges. However, it is not known to have been applied to systems that operate with set-up files pairing positioning sources and receivers in a timing sequence that avoids interference from reflection signals.